Thermocouple with nickel-silicon-magnesium alloy negative element

ABSTRACT

A nickel-base alloy, useful as a negative thermoelement, is disclosed. The alloy consists essentially of from about 3.25 to about 5.0 percent by weight of silicon and from about 0.03 to about 0.25 percent by weight of magnesium and the balance nickel. The new alloy is more oxidation resistant and has a greater EMF stability than existing type K negative thermoelements.

United States Patent 1 Starr et a].

[451 Dec. 4, 1973 THERMOCOUPLE WITH NICKEL-SILICON-MAGNESIUM ALLOYNEGATIVE ELEMENT [75] Inventors: Carrol Dean Starr, Whippany; Teh P0Wang, North Caldwell, both of NJ.

[73] Assignee: Wilbur B. Driver Co., Newark, NJ.

[22] Filed: Apr. 12, 1973 [21] Appl. No.: 350,641

Related US. Application Data [62], Division of Ser. No. 173,671, Aug.20, 1971.

[52] US. Cl 136/239, 136/236, 75/170 [51] Int. Cl H01v 1/22 [58] Fieldof Search 136/236, 239;

[561 References Cited UNITED STATES PATENTS 1,076,438 10/1913 Marsh136/239 X 2,224,573 12/1940 Hunter 136/239 2,859,264 11/1958 Hunter136/239 2,990,440 6/1961 Obrowski et a1... 136/239 3,411,956 11/1968Sibley 136/236 3,673,003 6/1972 Starr et a1. 136/239 X FOREIGN PATENTSOR APPLICATIQNS 172,087 12/1965 U.S.S.R 136/239 Primary ExaminerLelandA. Sebastian Assistant ExaminerE. A. Miller AztorneyNorman J. OMalley etal.

[5 7 ABSTRACT 5 Claims, N0 Drawings THERMOCOUILE WITHNICKEL-SILICON-MAGNESIUM ALLOY NEGATIVE ELEMENT CROSS-REFERENCE TORELATED APPLICA- TION This application is a division of Ser. No.173,671, filed Aug. 20, 1971 and assigned to the assignee of the presentinvention. Original assignment recorded on Aug. 20, 1971, Reel 2775,Frame 135. BACK- GROUND OF THE INVENTION This invention relates tonickel-base alloys. More particularly, it is related to'nickel-basealloy having a superioroxidation resistance and EMF stability under longtime periods of usage than existing type K negative thermoelements.

Nickel-base alloys have been in use as thermoelements since the earlypart of this century. One of the commonly used thermocouples is the typeK thermocouple (designated by the Instrument Society of America). Thepositive type K thermoelement is a nickel base alloy containing 9.25 Cr,0.4 Si, balance essentially nickel. The negative type K thermoelement isa nickel base alloy containing'ZAl, 2.5 Mn, 1 Si, with small amounts ofFe and Co. There are numerous commercial suppliers of type Kthermocouples.

The type K thermocouple is recommended to be used in air atmosphere. Athigh temperatures in air the negative type K thermoelement is the weakerlink of the couple. It fails much sooner than the positive type Kthermoelement because of its poorer oxidation resistance. In addition, amuch larger portion of the EMF drift of the couple as a whole isattributed to the EMF change of the negative thermoelement. As a result,

there is a need for a new negative thermoelement with improved oxidationresistance and EMF stability.

It is believed, therefore, that a new alloy useful as a negativethermoelement which has excellent oxidation resistance and EMF stabilityis an advancement in the art.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of this inventionto provide a new alloy which is oxidation resistant.

It is a further object of this invention to provide an alloy havingexcellent EMF stability.

It is a further object of this invention to provide a negativethermoelement alloy.

These and other objects are achieved in one aspect of this invention byan alloy consisting essentially of from about 3 .25 percent to about 5.0percent by weight of silicon, from about 0.03 percent to about 0.25percent by weight of magnesium and the balance nickel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS the thermocouple hasbecome more important than the actual EMF at a particular temperature.It has been found that the alloys of this invention change very littleeven after being exposed in air from over 21,000 hours at l,800 F. Whencompared to the negative thermoelement now in use, the error in athermocouple is greatly reduced.

Addition of silicon to nickel is known to improve oxidation resistanceover the thermoelements now in use, hence longer life; however, thealloys of this invention containing magnesium exhibit better EMFstability than alloys without the magnesium.

The alloys of this invention consist essentially of from about 3.5 toabout 5 percent by weight of silicon and from about 0.03 percent toabout 0.25 percent by weight of magnesium and the balance nickel.Preferred are alloys containing from about 3.75 to about 4.25 percent byweight of silicon and from about 0.10 to about 0.20 magnesium withbalance of nickel. Especially preferred is an alloy containing 4%silicon, 0.15 percent magnesium and balance nickel.

EXAMPLE 1 A. Individual Thermoelement EMF at 1800 F, vs. Pt MV EMFInitial After 21,196 Hr. in MV positive type K thermoelement negativetype K thermoelement B. Thermocouple After 21,196 Hrs.

EMF in MV Temp. in F 1 K vs. K +0549 +25 2 K vs. Ni/4Si +0403 +18 3 Kvs. Ni/4Sl/0.15Mg +0.167 +7 (1) For a type K couple the thermoelectricpower at 1800" F is 0.022 MV per F and the other two thermocouples (2and 3) the thermoelectric power is 0.023 MV per F.

The above long term atest at a high temperature of l,800 F decisivelydemonstrates that the Ni/4Si/0.15 Mg alloy is far superior to the othersto both the conventional type K thermoelement and to a 4 percentsilicon-nickel alloy.

The beneficial effect in EMF stability is attributed to the addition ofthe proper amount of magnesium to the Ni/4Si alloy base.

The range of magnesium content is from 0.03 to about 0.25 percent Mg,with the preferred amount of from 0.10 to 0.20 percent Mg. The presenceof Mg in nickel in trace amounts of 0.01 percent or below does not haveany beneficial effect on EMF stability and the presence of Mg above 0.25percent impairs the workability of the alloy.

The range of silicon content is from 3.25 to 5.00 percent. A minimum ofabout 3.25 percent is needed for oxidation resistance. Above about 5percent Si, the workability of the alloy is impaired. Preferred is fromabout 3.75 to about 4.25 percent silicon.

While there has been shown and described what is at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:

1. A thermocouple consisting essentially of apositive thermoelementalloy consisting essentially of about 9.25 percent chromium, 0.4 percentsilicon and balance nickel and a negative thermoelement alloy consistingessentially of from about 3.25 to about 5.0 percent by weight of siliconand from about 0.03 to about 0.25 percent by weight of magnesium andbalance nickel.

2. A thermocouple according to claim 1 wherein in said negativethermoelement alloy said silicon is from about 3.75 to about 4.25percent by weight.

3. A thermocouple according to claim 1 wherein in said negativethermoelement alloy said magnesium is from about 0.10 to about 0.20percent by weight.

4. A thermocouple according to claim 2 wherein in said negativethermoelement alloy said magnesium is from about 0.10 to about 0.20percent by weight.

5. A thermocouple according to claim 4 wherein in said negativethermoelement alloy said silicon is about 4 percent by weight and saidmagnesium is about 0.15

percent by weight.

2. A thermocouple according to claim 1 wherein in said negativethermoelement alloy said silicon is from about 3.75 to about 4.25percent by weight.
 3. A thermocouple according to claim 1 wherein insaid negative thermoelement alloy said magnesium is from about 0.10 toabout 0.20 percent by weight.
 4. A thermocouple according to claim 2wherein in said negative thermoelement alloy said magnesium is fromabout 0.10 to about 0.20 percent by weight.
 5. A thermocouple accordingto claim 4 wherein in said negative thermoelement alloy said silicon isabout 4 percent by weight and said magnesium is about 0.15 percent byweight.